big problem, please help

I breed my own rats too and have had this happen on occasion. Good healthy female with babies, access to food and water 24/7 who turns up really skinny.

Her I whack and toss, the babies are fed off. Don't need those genes in the pool.

It doesn't happen often but it does happen. Some are just not good at motherhood.

You do have to watch the smaller built females when their litters are getting close to weaning age, especially if she's got a decent size litter with her. They'll hog the water bottle but then nurse as much off her as possible so without careful management some of these females run down fast. Usually if I have a newer female or one that's not as big and sturdy as some of them, I'll use her litter to feed from when I need pinks or fuzzies or foster some of them off to another female that can more easily handle big demanding pups.